checkout: inform the user when removing branch state

After a successful switch, if a merge, cherry-pick or revert is ongoing,
it is canceled. This behavior has been with us from the very early
beginning, soon after git-merge was created but never actually
documented [1]. It may be a good idea to be transparent and tell the
user if some operation is canceled.

I consider this a better way of telling the user than just adding a
sentence or two in git-checkout.txt, which will be mostly ignored
anyway.

PS. Originally I wanted to print more details like

    warning: cancelling an in-progress merge from <SHA-1>

which may allow some level of undo if the user wants to. But that seems
a lot more work. Perhaps it can be improved later if people still want
that.

[1] ... and I will try not to argue whether it is a sensible behavior.
There is some more discussion here if people are interested:
CACsJy8Axa5WsLSjiscjnxVK6jQHkfs-gH959=YtUvQkWriAk5w@mail.gmail.com

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -337,11 +337,14 @@ void create_branch(struct repository *r,
 	free(real_ref);
 }
 
-void remove_branch_state(struct repository *r)
+void remove_branch_state(struct repository *r, int verbose)
 {
-	unlink(git_path_cherry_pick_head(r));
+	if (!unlink(git_path_cherry_pick_head(r)) && verbose)
-	unlink(git_path_revert_head(r));
+		warning(_("cancelling a cherry picking in progress"));
-	unlink(git_path_merge_head(r));
+	if (!unlink(git_path_revert_head(r)) && verbose)
+		warning(_("cancelling a revert in progress"));
+	if (!unlink(git_path_merge_head(r)) && verbose)
+		warning(_("cancelling a merge in progress"));
 	unlink(git_path_merge_rr(r));
 	unlink(git_path_merge_msg(r));
 	unlink(git_path_merge_mode(r));

branch.h

@@ -64,7 +64,7 @@ extern int validate_new_branchname(const char *name, struct strbuf *ref, int for
  * Remove information about the state of working on the current
  * branch. (E.g., MERGE_HEAD)
  */
-void remove_branch_state(struct repository *r);
+void remove_branch_state(struct repository *r, int verbose);
 
 /*
  * Configure local branch "local" as downstream to branch "remote"

builtin/am.c

@@ -1957,7 +1957,7 @@ static int clean_index(const struct object_id *head, const struct object_id *rem
 	if (merge_tree(remote_tree))
 		return -1;
 
-	remove_branch_state(the_repository);
+	remove_branch_state(the_repository, 0);
 
 	return 0;
 }

builtin/checkout.c

@@ -899,7 +899,7 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 				delete_reflog(old_branch_info->path);
 		}
 	}
-	remove_branch_state(the_repository);
+	remove_branch_state(the_repository, !opts->quiet);
 	strbuf_release(&msg);
 	if (!opts->quiet &&
 	    (new_branch_info->path || (!opts->force_detach && !strcmp(new_branch_info->name, "HEAD"))))

builtin/rebase.c

@@ -1272,7 +1272,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		if (reset_head(NULL, "reset", NULL, RESET_HEAD_HARD,
 			       NULL, NULL) < 0)
 			die(_("could not discard worktree changes"));
-		remove_branch_state(the_repository);
+		remove_branch_state(the_repository, 0);
 		if (read_basic_state(&options))
 			exit(1);
 		goto run_rebase;
@@ -1292,7 +1292,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			       NULL, NULL) < 0)
 			die(_("could not move back to %s"),
 			    oid_to_hex(&options.orig_head));
-		remove_branch_state(the_repository);
+		remove_branch_state(the_repository, 0);
 		ret = finish_rebase(&options);
 		goto cleanup;
 	}

builtin/reset.c

@@ -420,7 +420,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 			print_new_head_line(lookup_commit_reference(the_repository, &oid));
 	}
 	if (!pathspec.nr)
-		remove_branch_state(the_repository);
+		remove_branch_state(the_repository, 0);
 
 	return update_ref_status;
 }

builtin/revert.c

@@ -196,7 +196,7 @@ static int run_sequencer(int argc, const char **argv, struct replay_opts *opts)
 	if (cmd == 'q') {
 		int ret = sequencer_remove_state(opts);
 		if (!ret)
-			remove_branch_state(the_repository);
+			remove_branch_state(the_repository, 0);
 		return ret;
 	}
 	if (cmd == 'c')